Electron velocity modulation tubes



Se t. 17, 1963 D. E. c. LAMBERT ETAL 3,104,342

ELECTRON VELOCITY MODULATION TUBES Filed Oct. 29, 1959 7 Sheets-Sheet l Inventors: DONOVAN E. LAMBERT ERIC H. GROVER Attorney Pf- 7, 1 963 D. E. c. LAMBERT ETAL 3,104,342

ELECTRON VELOCITY MODULATION TUBES Filed Odt. 29, 1959 7 Sheets-Sheet 2 lnverdors: DONOVAN E. LAMBERT ERIC H. GROVER Attorney P 1963 D. E. c. LAMBERT ETAL 3,104,342

ELECTRON VELOCITY MODULATION TUBES 7 Sheets-Sheet 3 Filed Oct. 29, 1959 ventors: DONOVAN E. LAMBERT VER ERIC H. GRO

Attorney Sept. 17, 1963 D. E. c. LAMBERT ETAL 3,104,342

ELECTRON VELOCITY MODULATION TUBES Filed Oct. 29, 1959 7 Sheets-Sheet 4 I Inventors:

DONOVAN E. LAMBERT ERIC H. GROVER Byz A ttorn y Sept. 17, 1963 D. E. 'c. LAMBERT ETAL 3,104,342

ELECTRON VELOCITY MODULATION TUBES Filed Oct. 29, 1959 '7 Sheets$heet 5 Inventors; DONOVAN E. LAMBERT ERIC H. GROVER Attorney P 1963 D. E. c. LAMBERT ETAL 3,104,342

ELECTRON VELOCITY MODULATION TUBES Filed Oct. 29, 1959 7 Sheets-Sheet 6 ILL Inventors:

DONOVAN E. LAMBERT .ERIC H. GROVER WM l A Home y p 1963 D. E. c. LAMBERT ETAL 3,

ELECTRON VELOCITY MODULATION TUBES Filed Oct. 29, 1959 7 Sheets-Sheet 7 Inventors: DONOVAN E. LAMBERT ERIC H. GROVER ELECTRON VELGCETY MODULATION TUBES Donovan Ernest Charles Lambert and Eric Houghton Grover, London, England, assignors to International Standard Electrie Corporation, New York, N.Y.

Filed Get. 29, 1959, Ser. No. 349,530 Claims priority, application Great Britain Nov. 14, 1958 19 tliaims. (til. 3155.51)

The present invention relates to electron velocity modulation apparatus in which a beam of electrons is projected across a waveguide and is concerned with the construction of the electron tube comprising the beam generating electrodes and the section of waveguide across which the beam is projected. The invention is also concerned with an electron velocity modulation oscillator the practical embodiment of which has resulted in the aforementioned tube construction.

The basic principle of operation of the invention is that during part of its passage across the waveguide the electron beam is shielded from the electro-magnetic field of the Waveguide by being contained in a drift tube. At each end of the drift tube there are interaction gaps, as in the conventional klystron, so that the beam, when it crosses the finst gap, becomes velocity modulated; in the drift tube the velocity modulation becomes converted into density modulation, and at the second gap, at the other end of the drift tube, the bunched beam delivers up energy again to the field of the waveguide, so that, if the waveguide is made of a resonant length, oscillations of the resonant frequency will be generated. Suggestions have previously been made for this kind of device to be provided with an elongated beam of electrons lying along the length of the waveguide so as to be able to utilise a considerable length of cathode; this length can then be of the order of one quarter of a wavelength of the oscillating field. The guide wavelength depends, in the case of a rectangular guide, upon the relevant dimensions of the sides. For the dominant transversal electric, or TE, mode, in which the electric vector is perpendicular to one pair of opposite side, the guide wavelength is determined by the separation between the other pair of opposite sides and, as is well known, when these are brought closer together the cut-off frequency is raised; as the cut-off frequency approaches the operating fre uency the guide wavelength becomes longer and longer. It has therefore been proposed, in apparatus of this kind, to dimension the Waveguide so that it is operating near cut-off frequency and therefore giving as long a guide wavelength as possible, so allowing a longer electron beam to be used. In practice, however, it is seldom that a fixed frequency oscillator is required and provision has to be made for adjusting the length of guide so that it can be made resonant over a band of frequencies. If one is operating near the cut-off frequency it is evident that the guide wavelength will change rapidly with tuning. This is generally undesirable and may explain why tubes designed on these principles have not hitherto met with great success. In the present invention, which uses a ribbon-shaped beam, instead of placing the length of the ribbon along the direction of propagation of the waves, the length of the ribbon is placed across the waveguide that is, it is oriented in a direction which is independent of the guide wavelength so that the efiiciency of the device is much less critical upon the tuning than in those of the prior art.

For the successful practical embodiment of an oscillator according to the invention a special valve construction is needed which incorporates within the vacuum envelope the section of waveguide in which interaction with the beam takes place, and also, of course, the electrodes generating the electron beam and defining its path.

To this end a construction of tube according to the present invention comprises a box-like container in which the beam-forming electrodes, a short length of waveguide, the drift tube and other members are mounted as a unitary assembly from one end of the box, which end may conveniently comprise a conventional valve base on which the various electrodes are mounted as in conventional single ended valves, the valve pins being utilised to provide the necessary current and potential leads. The resonant length of Waveguide within the box is aligned with apertures in opposite sides thereof and these communicate through waveguide Windows with a further mechanical tuned, waveguide resonator and with a waveguide output feed arrangement outside the vacuum enclosure. The waveguide windows are inserted in respective cups mounted in a pair of waveguide flanges to which cups the aforementioned b0x-iike enclosure is also secured. Arrangements are made for mounting a focusing magnet for the electron beam between the same pair of waveguide flanges.

One of the main troubles with any construction of tube operating on the principles outlined above is that of preventing the generation of spurious modes of oscillation. The combination of drift tube and waveguide is apt to behave as a coaxial line, with the drift tube forming the inner conductor and the sides of the waveguide the outer conductor. In embodiments of the present invention special precautions are taken to ensure that spurious modes of oscillation are not generated within the operating range of the tube and this is achieved, not only by careful dimensioning of the drift tube with respect to the waveguide, but also by the adoption of a type of strip transmission line through which connection is made to the drift tube. Means are also employed for damping any spurious oscillations which may nevertheless occur.

The invention will be described in fuller detail with reference to the accompanying drawings in which:

FIG. 1 is a diagram of a resonant length of rectangular waveguide showing the variation of electric field in the guide;

FIGS. 2 and 3 are diagrammatic transverse and longitudinal cross-sectional illustrations, respectively, of the oscillator arrangement according to the invention;

FIG. 4 is a perspective view of a practical embodiment of an electron beam tube according to the invention;

FIG. 5 is an exploded view of the envelope assembly of the tube of FIG. 4;

FIG. 6 shows a detail of the assembly of FIG. 5;

FIG. 7 is a perspective drawing of the valve mount carrying the beam-forming electrodes and resonator of the tube of FIG. 4;

FIG. 8 shows the assembly of the valve mount of FIG. 7 in the envelope assembly of FIG. 5;

FIG. 9 illustrates a detail of the assembly of FIG. 8;

FIGS. 10 and ill are, respectively, plan and endelevational views of the tube of FIG. 4, FIG. 11 illustrating the focusing magnet adjustment;

FIGS. 12,, I3 and 15 are, respectively, a. partly cutaway perspective view, a longitudinal cross-section and a detail in cross-section, of the resonator assembly forming part of the valve mount of FIG. 7;

FIG. 14 is a perspective view, in part section, of the drift tube of FIGS. 12 and 13; and

FIGS. 16 and 17 are, respectively, perspective views of the tuner and waveguide impedance-matching coupler employed with the tube of FIG. 4 to provide an oscillator according to the present invention.

In order to make clear the principle of the invention a length of rectangular waveguide is illustrated in FIG. 1, with co-ordinate axes x, y, and z indicated at one corner. For propagation in the TE mode, the direction of propagation is along the z axis and the electric plane JKLM of FIG. 1.

vector, represented by the arrows E, is everywhere parallel to the y axis. The guide wavelength A is determined by the width dimension 2a. Wave propagation in this mode will occur at frequencies above a certain cut-off frequency which is determined by the dimension 2a, the guide wavelength h at the cut-off frequency being equal to Zn. Assuming a length of guide one-half a guide wavelength long, the intensity of the electric field varies sinusoidally along the length and across the width of the guide as is illustrated by the half sine waves and the arrows E, in the mid-plane ABCD along the length of the guide and in the transverse plane JKLM half-wave along the guide. At AD and BC, as also at JM and KL, there are nodes of the electric field; the two ends of the guide at AD and at BC may therefore be short-circuited so as to form a resonator a half guide wavelength long. It is evident that if an electron beam be projected across the guide in the plane ABCD, from,

say, the line CD to the line AB, interaction between the beam electrons and the electric field of the resonator may occur. Considerations of efificiency of any such system dictate that the width of the electron beam in the plane ABCD shall not exceed about a quarter guide wavelength, the electron beam being positioned about the centre of the resonator. Similarly the thickness of the beam in the plane JKLM should not exceed about half the width of the resonator. In practical devices the electron beam is invariably thin. The width of a ribbon-shaped beam lying in the plane ABCD is obviously determined by the guide wavelength A If one operates near the cut-off wavelength, then A may considerably exceed the free space wavelength and so a long cathode can be used. However, it would appear, both from our own experience and the absence of prior commercial tubes, that work along these lines, i.e. having an electron beam in the plane ABCD, has not been altogether successful. The reasons for this lack of success are not altogether understood, but may well be connected with the difliculties of tuning and the consequent variation of the width of the electron beam in terms of the resultant guide wavelength. In embodiments of the present invention, however, the electron beam is located in the plane JKLM. The dimensions of the beam are then virtually independent of variations in the guide wavelength due to tuning and an adequate width of beam may still be employed.

To use the resonant waveguide system of FIG. 1 as an oscillator, a drift tube is incorporated so that in their travel across the guide the beam electrons cross a first gap in which they are velocity modulated by the electric field of the guide, are permitted to drift and to bunch in the drift tube, and then deliver up energy to the electro-magnetic field in crossing a second interaction gap at the further end of the drift tube. Since the electric vector vanishes at the side walls JM and KL of the guide, the drift tube can be supported on these walls. Alternatively the drift tube can be insulated from the walls of the waveguide and used to impart frequency modulation by varying the drift tube potential in accordance with a modulating signal.

The basic arrangement in embodiments of the present invention is illustrated in FIGS.- 2 and 3. FIG. 2 represents, diagrammatically, a cross-section through the resonant waveguide in a plane corresponding to the FIG. 3 is a longitudinal crosssection corresponding to the plane ABCD of FIG. 1 but extending beyond the basic resonant length of guide to include a tuning section at one end and an output counting section at the other; FIG. 3 includes all those circuit components which determine the frequency of the oscillator. In FIGS. 2 and 3, the waveguide corresponding to that of FIG. '1 is represented at 1. An elongated slot 2 is formed in one wall of the waveguide, the slot having its length in the x-direction. An electron gun comprising a cathode 3, and, for the sake of generality,

side walls of the guide, but as has been stated above,

for frequency modulation purposes it may be insulated from the guide and mounted by means not shown and external to the waveguide. The length of the drift tube (in the y direction) is determined by the electron transit angle, i.e. by the mean electron velocity, normally set by the voltage difference between the cathode and the waveguide and drift tube. To provide interaction gaps of an optimum length in the y direction it is therefore convenient to provide fins 8 projecting from the. wave-' guide wall in alignment with the edges of the drift tube.

Referning to P16. 3, a short-cirouiting piston 9 adjusts.

the length of an additional waveguide resonator coupled to the basic resonant length of waveguide through a reactive diaphragm 10. This diaphragm is indicated as being of the inductive type, although capacitative coupling through a capacitative diaphragm could also be used. At the other end of the basic resonator section of waveguide 1 another diaphragm 11 affords coupling to a waveguide output circuit. The waveguide 1 is hermetically sealed off by waveguide windows 14 and 14 from the rest of the waveguide system, other envelope means, not shown, completing the vacuum enclosure for the electron beam system. The tuning piston 9 is carried in a tuner attachment comprising a length of waveguide 82 secured to flange 12 on waveguide 1. On the output side a coupler comprising a length of waveguide 91 fitted with a capacitative impedance-matching slug 95 is secured to the flange 13 on waveguide 1. The poles of a beam focusing magnet 20 are indicated at either end of the electron beam path.

7 From the radio frequency point of view the arrangement of FIG. 3 can be divided into three sections A, B and C, as indicated in the drawing. The section A, ex-

tending approximately between the diaphragms 10 and 11,

gether with the diaphragm 1-1, is equivalent to an output 1 The waveguide windows 14 and 14' are equivalent to additional capacitances loading the coupling transformer.

respective sections B and C, and, together with the change of shape of the waveguide in the region of the window holding cups, constitute very short lengths of complex transmission line. The section B forms a coupled circuit working in the fundamental mode with somewhat less than a quarter wavelength from diaphragm 10- to tuning piston 9. Diaphragm 10 is proportioned to provide fairly strong coupling between sections A and B to enable sufficient mechanical tuning to be obtained. Diaphragm 11 is proportioned to provide enough coupling to over couple a matched output waveguide to section A; the, coupling is then reduced to its optimum value by means of the capacitative slug 95.

In FIG. 3 the waveguide windows 14 and 14' are shown at approximately equal distances from the respec--' tive diaphragms 10 and 11, with rather a long sectionof the waveguide 1 between diaphragm and adjacent window. In order that the tuning piston 9 shall be able to move through as much as possible of the section B, it is obviously desirable that the distance between 10 and 14 be short. In our practical embodiments the spacing between flanges 12 and 13 is determined largely by the thickness of the magnet 20, which is fitted between these flanges; it follows that in the practical construction to be described,

the window 14 and the flange 12 are positioned close to the diaphragm 10, while window 14 and flange 13 are more widely spaced from diaphragm 11. The diaphragms and 11 are, of course, equidistant from the mid-plane of the electron beam, which means that the section A, FIG. 3, is located towards the tuner end of waveguide 1.

Although basically the arrangement of FIGS. 1 to 3 appears to be very simple, from a practical point of view there are considerable difiicul-ties. In the first place, as already mentioned trouble has been found in suppressing spurious modes of oscillation. It has been assumed, so far, that only the TE mode of rectangular waveguide propagation is involved, but, in fact, great care is needed to avoid the electron beam exciting other possible modes. Thus the resemblance is quite apparent between FIG. 2 and the well-known coaxial line type of oscillator tube in which a beam of electrons is projected across a resonant length of coaxial line through a passage in the inner conductor which acts as a drift tube. These unwanted modes are made evident, not so much by their appearance in the output of the oscillator, but by them taking energy from the desired mode so that when an oscillator exciting the T13 mode is tuned over a given range the graph of output power versus frequency may show dips or even has regions in which no oscillation is obtainable. The means adopted to overcome these difiiculties in embodiments of the present invention will be described further on.

A practical embodiment of a tube construction according to the invention is shown in perspective view in FIG. 4. The principal features visible in FIG. 4 are a pair of waveguide flanges 12 and 13 in which are positioned respective waveguide windows of which one is seen at 14 mounted in a cup 15 secured to the flange 12. A set of struts 16 maintains the spacing between waveguide flanges 12 and 13. A conventional form of valve base 17 is a prominent feature of the drawing of FIG. 4. This valve base is mounted in a cup 18 forming an extension at the upper end of an envelope assembly 19 of generally rectangular shape which contains the electron beam generating electrodes and a section of waveguide having window 14 at one end and a similar window at the other. Also visible is part of a permanent magnet 29 which serves to focus the electron beam and is clamped in position between the two waveguide flanges 12 and 13. A cylindrical projection 21 from the side of the box 19 is a cap protecting the exhaust tabulation for the evacuated envelope of the device. Other details indicated in FIG. 4 will be referred to below in the course of the description of the several component parts of the valve.

An exploded view of the exterior parts of the envelope assembly 19 of FIG. 4, is shown in FIG. 5 as it might be seen prior to putting together in various component parts. The assembly comprises a hollow non-magnetic metal box 22 of generally rectangular cross-section, closed at its lower end 23, and to whose open upper end is secured the extension cup- 18 which receives the valve base 17 of FIG. 4. Two opposite sides of the box 22 have aligned rectang-ular openings 24 towards the lower end of the structure. A pair of cups 15', similar to and receiving the pair of cups 15 of which one is seen FIG. 4, have base apertures 25 in alignment with the openings 24. The aperture 25 has lips, not visible in FIG. 5, which engage with the walls of opening 24 and over which is placed a spacing member 26. Members 27, 28 and 29 are rings of solder for securing together the component parts of the assembly. The method of assembly of the cup 15' to the box 22 is shown in the detail sketch of FIG. 6. A copper tubulation assembly 39, to be sealed otf after evacuation of the completed valve, is secured in an aperture 31 by means of solder ring 29.

The box assembly 19 of FIGS. 4 and 5, as explained above, serves as the vacuum envelope for the electron beam with its beam forming electrodes and the section of waveguide across which the beam is to be projected. This section of waveguide and the beam forming electrodes are mounted together as an integral assembly from the valve base -17 to form a mount, the principles of assembly of which are not unlike those employed in conventional radio receiving valves and the like. This mount is shown in FIG. 7, which is inverted with respect to FIG. 4. The valve base 17 is of the conventional type in which 8 pins and a central spigot are sealed into a glass button inset in a metal collar 32. Leads from the base passing through respective insulators 33, 34, 35 and 36 support the electrodes in conventional manner. A conventional getter 37 is mounted between the metal collar 32 and the central spigot 38. The section of waveguide together with the drift tube and the diaphragms which have been mentioned earlier is formed as a sub-assembly which we shall, for convenience, refer to as the resonator and which is indicated generally at 39. A prominent and important feature of the resonator assembly is a coo-ling block 40 which contains a cha-mfered recess Ed-in fact, for convenience in mounting the block 40 on an assembly jig, a hole is drilled and tapped to receive a countersunk screw, and the countersunk portion of the hole providing the recess 11. This recess is eventually filled with a solder ring which joins the mount to the base 23 of the box 22 of FIG. 5. An enclosure 42 mounted against the side of the resonator contains the cathode and control grid of a screen-grid type of electron gun. On the other side of the resonator an electron collector electrode is mounted in generally similar manner to that shown for the electron gun electrodes. The detailed arrangement of the electron gun and collector electrode mounting need not be discussed further as they are entirely similar to that adopted in coaxial line velocity modulation oscillator tubes where the resonator is a length of coaxial line and the drift tube is a passage through the inner conductor of the line. Before leavingFIG. 7 attention is directed to the four sheet metal foils 43 projecting like wings from the sides of the resonator assembly 39. These toils form part of the assembly of the waveguide windows after the mount has been inserted in the box 22 of FIG. 5. Before describing the details of the resonator assembly it is proposed to complete the description of the external construction of the tube.

After assembly of the mount of FIG. 7 the next stage of construction is the fitting of the mount into the envelope assembly 19, putting in the waveguide windows and then outgassing-the vacuum envelope by heating, activating the cathode and sealing ofi, very much as in conventional valve manufacture. This stage of the manufacture is illustrated in FIG. 8.

At the stage illustrated in FIG. 8 it is assumed that the envelope assembly 19 of FIG. 5 has been completed and that the mount of FIG. 7 has been inserted'into the box 22 and the valve base 17 has been soldered or welded into the cup 18, the exhaust tubulation 30 has been sealed to a length of glass tubing 44 for connection to the vacuum pumps. In FIG. 8 two of the toils 43 are seen folded over against the inside surface of the adjacent cup v15'. The member 45 is a serrated washer which seats against the foils 43 and also against the rear surface of the cup 15 which has sealed therein a rnica waveguide window 14. Sta-ted briefly, the arrangement is as follows: a piece of mica having serrated edges is sealed into the cup .15, this cup seats in the adjacent cup 15' with the serrated washer 45 and the foils 4-3 in between. The washer 45 is gold plated. The cups 15 and 15 are soldered or welded together along their adjacent outer rims to provide a vacuum seal. The base of the cup 15, around the mica window 14, is sufficiently flexible that on evacuation of the, air in the space between the cups 15 and 15' atmospheric pressure forces the surround of the mica window, the washer 45 and the foils 43 into intimate contact, so that when the assembly is heated during out-gassing the gold diffuses under pressure into the adjacent metal. and forms a gold ditfusion joint in known manner. The arrangement is illustrated in the enlarged detail view of FIG. 9. The reference numeral 46 indicates the end of the resonator 7 waveguide, which corresponds to waveguide 1 in FIG. 3. As shown, the foil 4-3 is secured to 46 and folded back against the inner surface of the cup 15. The cup 15 with its mica Window 14 seats against the washer 45 and, when the assembly operation is completed, the members 15, 45 and 43 are joined together by gold seals which ensure good high frequency contact through the window. Also during the heating process the solder ring filling the recess 41 in the cooling block 46 melts and solders the block to the base 23 of the box 22.

After processing of the valve on the pumps the glass tube 44 is first sealed off and then the copper tube 36 is squeezed off to form a cold welded tubulation in known manner. The protective cup 21 shown in FIG. 4 is then filled with solder and sealed over the end of the tube 30. The valve is now ready for assembly into the final form as shown in FIG. 4.

For the final stages of the assembly of the valve reference will be made once more to PEG. 4 and also to FIG. 10. It has been explained before that the waveguide flanges 12 and 13 are spaced from one another by means of the struts 16. Each of these struts 16 is in two parts, one part 47 is screwed into the other part 48 so that the strut length is adjustable, the flanges 13 and 12 being secured to the parts 47 and 48 respectively by means of screws such as the set 49 visible in FIG. 4. Before being secured to the struts 16, however, the flanges are secured to the respective cups 15 on each side of the Valve envelope portion. It will be seen from FIG. 8 that the cup 15 has a portion 50 of slightly larger diameter than the main cylindrical wall portion. The rim of the cup fits inside the waveguide flange but the flange and the cup are joined together through intermediate means comprising a clamping band 51 and a clamping ring 52 (FIG. The clamping band 51 is a thin metal member which fits closely about the wall of the cup and has a radial flange 53 which rests against the flange 12 or 13. Each clamping band is tightened up by means of a screw fastening 54, a set of screws 55, each of which is seated in the respective flange 12 or 13, passes through the flange 53 and is threaded into the clamping ring 52, and clamps the cup 15 to the waveguide flange 12 or 13, respectively. The two flanges 12 and 13 are then joined together by means of the set of struts 16.

The final assembly operation is the fitting and adjustment of magnet 20. This magnet, which is in the form of an incomplete ring having pole faces on each of the two opposite sides of the box 22 in alignment with the electron beam forming electrodes inside, is clamped between the waveguide flanges 12 and 13 by means shown in the side view of FIG. 11. V The magnet is held between a clamping plate 56 and a setting plate 57. The plate 56 extends over the whole of the width of the base of the magnet but the setting plate 57 bears only on the central portion of the base of the magnet. A pair of screws 58, seated in flange 12, are each threaded through the clamping plate 56 but are free to revolve in their seatings, and in clearance holes in the waveguide flange 13. The setting plate 57 is held against the magnet by means of a pair of set screws whose heads are visible in FIG. 4 at 59; they are threaded into the waveguide flange 12 and bear against the setting plate 57. The distance between the pole faces of the magnet and the sides of the box 22 can be adjusted; what is far more important, however, is the alignment of the magnetic field between these pole faces with the desired orientation of the electron beam. By means of adjustment of the screws 58 and 59 the magnet can be moved parallel to the waveguide flanges or can be given a slight tilt if required. This adjustment has to be performed while readings are taken of the electron beam current which is intercepted by the several electrodes of the beam system; it is therefore an adjustment normally performed in the factory and not touched again during the life of the valve.

The resonator 39' of FIG. 7 will now be more fully described. A diagrammatic perspective view of the resonator, from which the beam forming electrodes have axis, while the separation along the x axis between opposite sides determines the cut-oii wavelength of the guide and hence the guide wavelength. For the desired mode there should be no flow of current parallel to the x axis in practice unavoidable waveguide asymmetries will cause there to be some flow in the direction. For other, undesired, modes, particularly the dominant coaxial mode, the

main direction of current flow is parallel to the x axis, and the avoidance of excitation of such spurious modes by the electron beam is one of the major problems met in the design of this valve.

The body 60 of the resonator 39 is in the form of an open ended =box fabricated from the sheet metal. A cooling block 40 is secured in good thermal contact with the body 6! on one of the sides which lies in a yz plane. A drift tube 5, which consists essentially of two closely spaced parallel metal sheets lying in an xy plane, is clamped at one end to the cooling block 40 by means of a plate 61 and the other end of the drift tube, which passes outside the box, is positioned in an insulator 62 (FIG. 13). a

The width of the drift tube in the y direction is determined by the desired electron transit angle. Since this is, in general, considerably less than that provided by the physical distance between opposite sides of 60 at the ends of the y axis, as has already been explained with reference to FIG. 3, fins 8, of which one is visible in FIG. 12, are positioned in alignment with the drift tube so as to provide gaps at which the electron beam and the RT. electric field of the resonator interact, one of these gaps being indicated at 63.

The inductive diaphragm 10 is positioned in one end of the resonator body 60, the inductive diaphragm 11 is positioned approximately in the middle of the body and the drift tube and fin arrangement is placed midway between the two diaphragms 10 and 11. As had'been explained in connection with FIG. 3, the basic resonant length of waveguide extends between the diaphragms 10 and 11; to be precise, the eifect of these diaphragms, together with the tuner section B and the output coupling section C of FIG. 3, is to place efiective short circuits across the waveguide at respective planes slightly to the left and right, respectively, of the planes of 10 and 11 in FIG. 13, so that .the section A of FIG. 3 extends slightly beyond the diaphragms 10 and 11. Strictly speaking, this section A is the resonator of the velocity modulation oscillator, the body portion 60 of the structure of FIGS. 12 and 13 including also part of the output coupling waveguide section C of FIG. 3 for the mechanical reasons already explained in connection with that drawing; nevertheless it is thought that no confusion should arise in referring to the mechanical structure of FIGS. 12 and 13 as the resonator.

One of the major problems in the design of this or a similar type of oscillator is that of conducting away the heat which is inevitably generated in the resonator. This problem isovercome in embodiments of the present invention by the provision of the comparatively massive cooling block 40 which is fixed not only to the resona tor but, in the final assembly of the valve, also to the exterior metal envelope, as has previously been described.

Anotherg'and related, problem is that of the thermal stability of the oscillator. For operation at a frequency of 7,000 mc./s., for a given beam accelerating voltage a change in the dimensions of the gap 63 by 0.001 inch 9 and a similar change inthe other interaction gap gives a change in the natural frequency of the resonator of 100 to 150 mc./s. It is therefore very important to maintain the gap width constant in spite of thermal changes. The desired constancy of gap dimensions is obtained in embodiments of the present invention by use of different metals for the body 69 and fins 8 to that used for fabrication of the drift tube 5, together with choice of the physical dimensions of the system having regard to the electron transit time requirements. The critical dimensions and operating data for a specific embodiment of the invention will be listed later; in this embodiment, the body 65 and fins 8 are made of a copper-nickel non-magnetic, alloy which is similar in composition to that used for resistance wires while the drift tube is made of molybdenum. With this arrangement a to centrigrade degree change in ambient temperature can be tolerated while preserving a short term stability, reckoned in hours, of i1 rnc./s. at an operating requency of 7,000 mc./s.

The construction of the drift tube, its method of mounting, and the arrangement for connecting a potential lead thereto are all of importance in preventing the excitation of spurious modes by the electron beam and for obtaining a wide working range of operating wavelengths. In the first place, for the desired transverse electric modes, high frequency surface currents flow across the drift tube in the y direction, whereas in the undesired modes surface currents flow along the drift tube in the x direction. If the resonator were perfectly symmetrical about the mid-line of the drift tube in the xy plane, no current in the desired mode would flow along the drift tube in the x direction; due to mechanical imperfections, exact symmetry cannot be attained and some current in the desired mode does, in fact, flow along the drift tube in the x direction. The introduction of straightforward damping within the resonator means for currents in the x direction would therefore lead to coupling between the desired and undesired modes, so .the latter cannot be suppressed in this way. instead, impedance changes are introduced to tune undesired coaxial modes outside the band of frequencies covered by the mechanical and voltage tuning ranges of the oscillator when working in the desired modes and wave damping is applied to the open end of the drift tube outside the resonator. The impedance changes are provided by the shaping of the drift tube.

The drift tube is made up of a laminated construction of pieces of sheet molybdenum, gold fused together, the form of construction being shown in FIG. 14. Two outer laminae 66 and 67, respectively, are spaced from one another by the laminae 6S and 69 at either end of the assembly and by a dividing bar 64 which divides the mid-portion of the drift tube into two hollowed out passages for the electron beam. This subdivision is to prevent radiation from the structure which would occur were the laminae as and 67 to be connected together only at their ends, At each end of the electron beam passage the drift tube is solid and, as shown,'is narrower than the portion through which flows the electron beam. At one extreme end 79 it is stepped down to fit into the insulator 62 of FIG. 13 and at the other end 71 it is formed for clamping to the cooling block 4% From the point of view of the unwanted dominant coaxial mode, the changes in drift tube width provide corresponding changes in the impedance of the coaxial line of which the drift tube forms the inner conductor. Taking into account these impedance changes, the length of the drift tube, i.e. the length parallel to the x axis, is carefully chosen so that when open-ended, the coaxial line is not resonant at any frequency at which oscillation might be excited by passage of the electron beam over the range of current and voltage conditions used for the excitation of the desired TE modes. As an additional precaution, wave damping is applied to the electrically open end of the coaxial line formed by the drift tube 5 and the resonator body 60. This is achieved by means of the insulator 10 62; this insulator is a collar of high loss ceramic material which is held over the end 7d of the drift tube by means of a spring member '72 which is clamped between two nuts 73 and 74 secured to a stud on the body 69,

In practical use it is desiredto apply frequency modulating voltages to the drift tube; a potential lead has therefore to be connected to it. During early tests of the oscillator considerable difficulty was found owing to the fact that this potential lead was liable to form, with various members of the tube construction, types of strip transmission line which, when coupled to the drift tube, absorbed power from the wanted modes of oscillation. These difficulties have been overcome by adopting a coaxial transmission line method of connecting the drift tube potential lead 79 as indicated in FIG. 13 and a strip line method of connecting the drift tube to the cooling block as indicated in the enlarged detailed view of FIG. 15.

The end 71 of the drift tube is clamped between the face of the cooling block 40 and a plate 61 between strips 75 and 76 of insulating material by means of screws 77, which pass through clearance holes 78 (FIG. 14) in the end 71 of the drift tube. The plate 61 and cooling block 4f) form a quarter wave open-ended strip line which rejects power at the operating frequency and prevents undesired absorption. In addition, a length of tape 79, providing the potential lead for the drift tube, is passed through the hole 869 in the end 71 of the drift tube and is clamped between 71 and the insulator '75. From its junction with the drift tube the tape 79 is led out to one of the leads on the mount of FIG. 7 through an insulated bushing 8-1 traversing the cooling block 40. The tape and block thus form a coaxial line.

With the aid of the above described steps for the suppression of spurious modes and absorbtion of power from the wanted mode it has been possible to obtain a tuning range of over 50 percent of the mean operating frequency using suitable tuning and coupling arrangernents.

For operation of the tube in circuit two external attachments are made as has already been explained in connection with FIG. 3. One of these is a short coupling .section of waveguide providing impedance matching between the tube and waveguide utilisation apparatus, and the other is a tuner assembly.

The tuner assembly is illustrated in FIG. 16 and comprises, essentially, a short length of rectangular waveguide 132 containing a central non-contacting circular piston 83. The piston '83 may be moved axially by means of the micrometer screw drive 84. The mouth of the waveguide 132 is surrounded by a circular metal plate which, together with a serrated waveguide-jointing washer 86 is clamped on a cylindrical extension 87 of a circular waveguide flange $8. The extension 87 mates with the inside of the waveguide cup 15 (FIG. 4), the flanges 83 and 12 being bolted together so that the serrated washer 86 is brought into tight contact with the base of the cup 15. The waveguide 82 is thus made to form an extension of the portion of waveguide within the evacuatedenvelope of the valve. The actual waveguide window 14 is slightly recessed with respect to the outer surface of the base of the holding cup 15. This means that it is possible to bring the face of the piston 83 beyond the mouth of waveguide 8% until it almost makes actual contact with the mica window 14.

The piston tuning arrangement of FIG. l6 is differentiated from the conventional non-contacting piston arrangement in that the face of the piston carries a capacitative disc. As with certain other types of non-contacting piston, the diameter of the piston shaft is made alternately large and small, each section being a quarter of a wavelength long at the mean operating frequency, so as to provide alternate quarter wavelength sections of low and high impedance respectively. The high and low impedance portions of the piston immediately adjacent the end disc are just visible at 89 and 90 respectively in FIG. 16. The effect of the capacitative disc, together with the remainder of the piston, is to place a virtual short circuit in front of the disc. Thus when the piston is moved forward until the disc is just short of the waveguide window, the virtual short circuit is placed on the vacuum side of the Window. Although the same effect could be produced by a normal short circuiting piston moved back through a half wavelength, it can be shown that such higher mode working restricts the electronic tuning range of the oscillator to a greater extent than does the capacitative disc arrangement. a

The output coupling waveguide section is illustrated in FIG. 17. it comprises a rectangular waveguide section 91 together with a waveguide jointing washer 92 and circular waveguide flange 93 so as to mate with the other waveguide flange 13, and window cup of the valve of FIG. 4 in generally similar manner to that decribed with reference to the tuner of FIG. 16. The waveguide 91, however, is of larger dimensions than 82 and is of the standard size for propagation of waves about the mean frequency of operation of the oscillator. The other end of the length of waveguide 91 is joined to a conventional rectangular waveguide flange 94 for coupling to the utilisation waveguide system. Near the front end of the waveguide 91 can be seen a capacitative post 95 whose penetration into the waveguide is adjustable by means of the knurled knob 96. By means of this coupler an optimum impedance match can be effected between the valve and the waveguide system in which it is used.

In a practical embodiment of the invention for operation over the frequency range 6,800 to 7,500 mc./s., the critical internal dimensions of the resonator 23 of FIG. 12 were as follows:

Resonator body 1 inch x 0.35 inch cross-section. Length of box 46 (without contact foils 32) Length of wide section of drift tube (parallel to x axis) Separation between outer drift tube plates (66, 67, FIG. 14) Drift tube width between interaction gaps Width of ends of drift tube 6'8, 69 (FIG. 14) (parallel to y axis) 0.080 inch. Width of gap 63, FIG.

12 0.017 inch. Overall length of drift tube 1.320 inches.

7 The tube is designed to operate with a maximum cathode current of 65 ma. and, for use in mode 15, i.e. the electron transit time in the drift tube being three and three quarter cycles, the drift tube voltage is about 450 volts with respect to the cathode. In mode 19 (four and three quarter cycles), the beam accelerating voltage is about 260 volts. The power output in mode 15 is then between 800 and 900 milliw'at-ts in the frequency range 6,8007,500 mc./s. In mode 19 the power output is reduced to between 260 and 320 milliwatts. By modulation of the drift tube, in mode 15 oscillation deviation between half power points of 17 to 19 mc./s., depending upon the mid-band frequency of operation, can be obtained. In mode 19 the corresponding electronic tuning range is between 12 and 16 mc./s. As mentioned previously, by means of the bimetallic construction of the resonator, excellent thermal frequency stability is obtained, the initial thermal drift from cold being between 9 and 13 mc./s. and completed within less than 0.94 inch.

.620 inch.

.025 inch.

0.242 inch.

12 minutes. The variation of frequency with ambient temperature varies over the frequency range coveredby movement of the tuning piston from 50 to kc./s. per degree centigrade.

Various modifications of the constructions described above will occur to those skilled in the art.

illustrated in FIG. 5, the output coupling arrangements could be modified by omitting the waveguide window on the output side and (taking the output from the tuner, 7

either by means, for example, of an output aperture in the tuning resonator or by provision of a leaky tuning piston. For such an arrangement the diaphragm 11 on the output end of the main resonator would bereplaced by a short-circuiting plate and suitable alterations would be required for the mounting and adjustment of the beam focusing magnet. The arrangement with the threeport envelope construction is at present preferred as it provides the flexibility of independent tuning and output coupling arrangements.

It has been mentioned that capacitative diaphragms could replace the inductive diaphragms 10 and 11; capacita-tive tuning and output coupling diaphragms have, in fact, been used experimentally with no fundamental alteration of the operation of the oscillator.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the inopening in one of its sides, a hollow rectangular waveguide surrounding said drift tube, a portion of said waveguide containing the said drifit tube aligned with the said opening, electron beam forming electrodes mounted within the box outside the waveguide, and a waveguide window hermetically sealing the said opening.

2. An electron velocity modulation tube according to claim 1 further comprising an electron collector electrode mounted to an inner surface of said box at a location outside the waveguide at the end of said box opposite the said beam forming electrodes.

3. An electron velocity modulation tube according to claim 1 in which there are two openings in opposite sides of the box, the said waveguide joins the two openings and each opening is hermetically sealed by a respective waveguide window.

4. An electron velocity modulation tube according to claim 3 in which the said waveguide windows are mounted in a pair of waveguide flanges mounted to said box at the openings thereof spaced from one another by a set of struts, the said flanges providing mechanical support for the said tube.

5. Electron velocity modulation apparatus comprising a velocity modulation tube according to claim 4 in which a beam focusing permanent magnet is supported between the said flanges within the set of struts.

6. Apparatus according to claim 5 in which the said magnet is held between a pair of clamping plates supported from the said flanges by adjustable screws to per-' mit a limited range of adjustment of the position of the magnet poles with respect to the electron beam path.

'7. An electron velocity modulation tube according to claim 4 in which each said waveguide window is sealed into a shallow cup secured in a said waveguide flange and to the base of which shallow cup is secured the adjacent wall of the said box.

Thus, in place of a three-port box envelope construction such as 8. An electron velocity modulation tube according to claim 7 in which the said one end of the box carries a circular cup in which is sealed a glass valve base.

9. An electron velocity modulation tube according to claim 8 in which the said portion of the waveguide containing the drift tube, the drift tube mounting means and the beam forming and collecting electrodes are mounted as a unitary assembly from the said valve base.

10. An electron velocity modulation tube according to claim 9 in which to the said portion of waveguide is secured a cooling block of metal mounted on the outside of one waveguide wall and joined in thermal contact with the said elongated box.

11. An electron velocity modulation tube according to claim 10 in which the said drift tube comprises a central portion apertured to surround the electron beam and two end portions projecting through opposite sides of the said portion of waveguide, the drift tube being secured to the said cooling block by one of the projecting end portions and insulated from the cooling block.

12. An electron velocity modulation tube according to claim 11 in which fin inserts apertured to surround the electron beam in alignment with the beam passage through the drift tube project into the respective other opposite sides of the said portion of waveguide to form with the adjacent drift tube surfaces the velocity modulating and energy extracting gaps, respectively for the electron beam.

13. An electron velocity modulation tube according to claim 10 in which the said drift tube is secured to the said cooling block by being clamped between a metal clamping plate and one surface of the cooling block with dielectric insulation between the two metal clamping surfaces and the drift tube, the drift tube and clamping plate so forming a quarter wave open ended length of strip transmission line, and in which a potential lead for the drift tube is joined to the drift tube within the said length of strip transmission line.

14. An electron velocity modulation tube according to claim 13 in which the said potential lead is a thin metal tape rigidly supported between the surface of the drift tube and the adjacent insulation and said metal tape extending through a hole in the drift tube and through an insulated bushing in the said cooling :block.

15. An electron velocity modulation tube according to claim 14 in which the projecting portion at the other end of the drift tube passes through a clearance hole in the waveguide wall and is seated in a collar of lossy dielectric material which serves to damp HF. currents of coaxial mode flowing in the drift tube.

16. An electron velocity modulation tube according to claim 15 in which the said drift tube is of laminar construction.

17. An electron velocity modulation tu'be according to claim 16 in which the said portion of waveguide includes a reactive diaphragm for coupling to a further length of waveguide beyond the evacuated said portion. 7

18. An electron velocity modulation tube according to claim 17 in which the said portion of Waveguide comprises, as part of a unitary assembly therewith, a pair of metal foils projecting from each end of the portion, said foils being rigidly supported in the transversely extending bases of respective metal cups which are sealed to the said elongated box andprovide mounting means for respective hermetic sealing waveguide windows.

19. An electron velocity modulation oscillator comprising a resonant length of hollow rectangular waveguide, means for passing an electron beam through slots in the opposite walls of said resonant length with the width of the beam normal to the direction of propagation of the TB waves in the said guide, a drift tube surrounding the electron beam within the guide, interaction :gaps at each end of said drift tube, and a set of fins aligned with said drift tube and projecting from the respective walls toward the drift tube, the said interaction gaps being formed between the respective ends of the drift tube and the adjacent set of fins, the electron passage within the enclosure of the fins and within the drift tube being subdivided by metallic partitions to prevent radiation from the drift tube and the fins aligned therewith, and the said partitions mounted within the drift tube in the path of said electron beam to subdivide the electron beam into a plurality of coplanar ribbons.

References Cited in the file of this patent UNITED STATES PATENTS 2,320,860 Fremlin June 1, 1943 2,408,409 Bowen Oct. 1, 1946 2,452,561 Gibson Nov. 2, 1948 2,485,661 Roach Oct. 25, 1949 2,727,180 Wheeler Dec. 13, 1955 

19. AN ELECTRON VELOCITY MODULATION OSCILLATOR COMPRISING A RESONANT LENGTH OF HOLLOW RECTANGULAR WAVEGUIDE, MEANS FOR PASSING AN ELECTRON BEAM THROUGH SLOTS IN THE OPPOSITE WALLS OF SAID RESONANT LENGTH WITH THE WIDTH OF THE BEAM NORMAL TO THE DIRECTION OF PROPAGATION OF THE TE WAVES IN THE SAID GUIDE, A DRIFT TUBE SURROUNDING THE ELECTRON BEAM WITHIN THE GUIDE, INTERACTION GAPS AT EACH END OF SAID DRIFT TUBE, AND A SET OF FINS ALIGNED WITH SAID DRIFT TUBE AND PROJECTING FROM THE RESPECTIVE WALLS TOWARD THE DRIFT TUBE, THE SAID INTERACTION GAPS BEING FORMED BETWEEN THE RESPECTIVE ENDS OF THE DRIFT TUBE AND THE ADJACENT SET OF FINS, THE ELECTRON PASSAGE WITHIN THE ENCLOSURE OF THE FINS AND WITHIN THE DRIFT TUBE BEING SUBDIVIDED BY METALLIC PARTITIONS TO PREVENT RADIATION FROM THE DRIFT TUBE AND THE FINS ALIGNED THEREWITH, AND THE SAID PARTITIONS MOUNTED WITHIN THE DRIFT TUBE IN THE PATH OF SAID ELECTRON BEAM TO SUBDIVIDE THE ELECTRON BEAM INTO A PLURALITY OF COPLANAR RIBBONS. 